Radio frequency (RF) power amplifiers are often used in an array of portable applications, such as cellular telephones and wireless network devices. In such applications, the operating voltages are limited to those available from a battery source. Operating voltages tend to decrease as the battery source is discharged. The use of low operating voltages presents performance challenges in the design of RF power amplifiers.
In RF power amplifiers, bias voltage and current provided to the base of amplifying transistor(s) determine the class of operation and the linearity of the RF power amplifier response. Conventional current mirror bias reference circuits that have low output impedance may be used to provide bias voltage and current, but are often more sensitive to variations in the operating voltages. Current mirror reference sensitivity to the operating voltage level is undesirable.
FIG. 1 illustrates one example of a bias reference circuit. A source reference voltage Vref is provided, typically from a battery. The current level through the resistor R1 is essentially determined by (Vref−Vbe)/R1, where Vbe is relatively constant. The current through the transistor Q1 acts to set Vbe where Vbe=Vt*In(Ie/Is), which is then essentially mirrored in the RF power amplifying transistor to be biased. The RF amplifying transistor to be biased tends to have the same current density as the reference transistor Q1 (both are biased with reference to ground), but because the RF amplifying transistor to be biased is a larger device, it has more current. The current through the resistor R1 (and hence substantially the same as through transistor Q1) is mirrored in the RF amplifying transistor to be biased, to provide the appropriate output signals.
In the case of a heterojunction bipolar transistor (HBT), Vbe tends to be approximately 1.25 volts, and Vref tends to be approximately 2.85 volts. Therefore, a decrease in Vref by 0.1 volts results in a change in the current level of approximately 7%. In many applications, variability in the current level of 7% is not acceptable. A highly regulated voltage Vref may be used to provide suitable control of the bias for the RF amplifying transistor.
FIG. 2 illustrates another bias reference circuit. A source reference voltage Vref is provided, typically from a battery. The current level through the resistor R1 is determined by (Vref−2*Vbe)/R1, where Vbe is relatively constant. The current through the resistor R1 is mirrored in the RF amplifying transistor to be biased, to provide the appropriate output signals.
In the case of a HBT, the Vbe tends to be approximately 1.25 volts (2.5 volts for 2*Vbe), and Vref tends to be approximately 2.85 volts. Therefore, a decrease in Vref by 0.1 volts results in a change in the current level of approximately 40%. In many applications, a variability in the current level of 40% is not acceptable. Again, highly regulated voltage Vref may be used to provide suitable control of the bias for the RF amplifying transistor.
FIGS. 3, 4, and 5 illustrate other bias reference circuits. A source reference voltage Vref is provided, typically from a battery. The current level through the resistor R1 is determined by Vbe and the resistors (R1, R2, R1A, R1B), where Vbe is relatively constant. The current is mirrored in the RF amplifying transistor to be biased, to provide appropriate output signals.
In the case of a HBT, Vbe tends to be approximately 1.25 volts (2.5 volts for 2*Vbe), and Vref tends to be approximately 2.85 volts. Therefore, a decrease in Vref by a small amount results in a significant current level change. Again, a highly regulated voltage Vref may be used to provide suitable control of the bias for the RF amplifying transistor.